1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to an absorberless phase-shifting mask (PSM) for EUV and a method of fabricating such an EUV mask.
2. Discussion of Related Art
During the fabrication of integrated circuits (IC), a semiconductor wafer is subdivided into contiguous fields that are essentially identical. Each field typically includes one or more IC chips. A radiation-sensitive material, such as photoresist, is applied over the wafer. Then a wafer stepper with reduction projection optics scans radiation across a mask and onto each field sequentially to expose portions of the photoresist. The mask determines the pattern to be transferred onto the wafer through the processes of exposing, developing, and etching.
Improvements in lithography have increased the density and enhanced the performance of IC chips. According to the Rayleigh criterion, the critical dimension (CD) that may be resolved by a wafer stepper is directly proportional to the wavelength of the illumination and inversely proportional to the numerical aperture (NA) of the projection optics. For example, deep ultraviolet (DUV) lithography uses a wavelength of 157 nanometers (nm), 193 nm, or 248 nm and a NA of 0.45-0.80 to print a feature with a CD in the range of 70-180 nm.
A feature with an even smaller CD may be printed using next generation lithography (NGL), such as extreme ultraviolet (EUV) lithography. An EUV wafer stepper may use four imaging mirrors and an NA of 0.10 to achieve a CD of 50-70 nm with a depth of focus (DOF) of 1.00 micrometer (um). Alternatively, an EUV wafer stepper may use six imaging mirrors and an NA of 0.25 to print a CD of 20-30 nm although the DOF will be reduced to 0.17 um.
A binary mask only modulates the amplitude of the exposure light. Thus, diffraction will degrade the fidelity of the printed feature as the CD to be printed approaches the wavelength of the exposure light. However, a phase-shifting mask (PSM) modulates the phase of the exposure light, in addition to the amplitude of the exposure light. Consequently, a PSM can resolve a feature with a CD that is about the same size as or smaller than the wavelength of the exposure light.
Thus, what is needed is a method of creating an absorberless phase-shifting mask (PSM) for EUV and a method of fabricating such an EUV mask.